EECS Seminar: Novel Optical Modulators

Abstract: Optical modulators are essential to transmit electrical information through an optical fiber, which is the medium of choice for many applications including fiberoptic communications, datacom networks, optical signal processing, RF photonics and instrumentation. Main attributes of optical modulators are the drive voltage, typically called V_Π, electrical and optical bandwidth, insertion loss, optical power handling capability and cost. V_Π is desired to be as low as possible to reduce the electrical power consumption. The electrical bandwidth needs to be as high as possible to take advantage of the ultrawide bandwidths offered by fiber optics enabling the delivery of massive amounts of data through the internet. In addition, a low insertion loss is highly desirable to keep optical power needed from an external source as low as possible.

Meeting these requirements creates conflicting design conditions and usually compromises have to be made. This work presents devices relaxing the conflicts in the design of optical modulators using substrate removal.

This technique allows the processing of both sides of a semiconductor epilayer and allows the fabrication of any conceivable design. In these designs, epilayer is kept only where it is needed and the gap of the modulator electrode is determined by the thickness of the epilayer. This allows for very uniform and narrow gaps resulting in significant V_Π reduction. Using very thin InP based epitaxial layers containing multiple quantum wells yielded V_Π as low as 0.2 V. This work was extended to yield modulators with V_Π less than 0.7 V and bandwidths more than 67 GHz. Mode transformers reducing the high insertion loss in coupling in and out of very compact optical waveguides used will also be described. The remaining attribute of optical power-handling capability requires material with bandgap larger than twice the energy of two photons to suppress two-photon absorption, which turns out to be a limiting factor for high-optical power handling. Modulators fabricated in AlGaAs material system demonstrated drive voltages as low as 1 V even under optical powers approaching 1 watt.

These devices can also be fabricated in III-V epilayers directly grown on silicon substrates potentially providing devices at lower cost.

Bio: Nadir Dagli received a doctorate in electrical engineering from MIT in 1986. After graduation, he joined theDepartment of Electrical and Computer Engineering at UC Santa Barbara, where he is currently a professor and chair. His current interests are design, fabrication and modeling of guided-wave components for optical integrated circuits, ultrafast electro-optic modulators, WDM components and photonic nanostructures. He has served and chaired many conferences, technical programs and other professional committees. He was the editor-in-chief of IEEE Photonics Technology Letters from 2000-2005 and an elected member of the IEEE-LEOS board of governors from 2003-2005. He is a fellow of IEEE and OSA.